1. Field of the Invention
The present invention relates to an optical memory photosensitive material of a DRAW (direct read after write) type and a photosensitive material of an ROM (read only memory) type. More particularly, it relates to a highly sensitive photosensitive material and an image forming method that employs said photosensitive material, for and in which writing is carried out by a photon-heat mode.
The present invention further relates to a photosensitive material capable of forming a polymer image by light and/or heat and an image forming method that employs said photosensitive material.
2. Related Background Art
Developments have been hitherto made on many types of optical recording mediums utilizing coherent laser beams. These optical recording mediums include optical discs exclusively used in ROM, such as video discs and compact discs, and DRAW mediums in which users can write only once.
As the DRAW mediums, those in which chalcogenide metal thin films are used have been developed in the early years, and besides these those in which organic dyes are used have now been made available through mass production. The writing of information in the DRAW mediums presently mass-produced is carried out by the so-called heat mode, in which, e.g., a recording layer is partially vaporized by irradiation with semiconductor laser beams or the like to form holes (or pits).
To carry out the writing by the heat mode as mentioned above, a "hollow structure" as shown in FIG. 20 is commonly employed from the viewpoint of sensitivity. Optical recording mediums having such a hollow structure are so constructed that a recording layer 31 is set inside and substrates 32 are partially fixed through a spacer 33 to form a hollow space 30, and therefore they have the disadvantages that no sufficient adhesion can be obtained between the respective members, resulting in a poor mechanical strength.
Mediums other than the above mass-produced DRAW mediums in which chalcogenide metals or organic dyes are used, include a DRAW medium comprising a recording layer (a silver thin film) formed by a wet treatment using a silver emulsion, as disclosed, for example, in Japanese Patent Application Laid-open Nos. 55-108995, 56-3399 and 56-49297. The writing of information in this DRAW medium is also carried out by the so-called heat mode in which the silver thin film is partially melted by irradiation with semiconductor laser beams or the like to form holes (or pits). Accordingly, the medium must have the "hollow structure".
Mediums other than the above optical recording mediums which has the hollow structure and in which writing is carried out by a heat mode, also include a medium comprising a recording layer containing a heat-developable dry silver salt (a photosensitive silver salt and a reducing agent), as disclosed, for example, in Japanese Patent Application Laid-open No. 57-171330. The recording using this recording medium is carried out by producing silver metal through exposure to light, causing oxidation-reduction reaction between the silver salt and the reducing agent by heating, and thus forming a silver image utilizing the silver metal as a catalyst where the recorded information can be read out utilizing a difference in the amount of the transmitted light at the silver image portion.
Hence, the recording medium employing the heat-developable dry silver salt is not required to have the hollow structure since no holes are formed, and is therefore advantageous in view of mechanical strength or cost.
Such a recording medium, however, requires a recording unit having both a light irradiation means and a heating means in carrying out the writing of information, resulting in a large-sized or complicated unit, and hence is not practical as the DRAW medium in which users carry out writing.
Moreover, since the medium has not particularly any substance that reflects read-out light, it is difficult to control the tracking, and also difficult to use semiconductor laser beams as read-out light.
In another aspect, such a recording medium employing the heat-developable dry silver salt has the disadvantage that sometimes the recorded information once written (such as dots formed of a silver image) can not be read out after a long period of time. This is responsible for the unauthorized reaction that may be caused after a long period of time between the silver salt and reducing agent remaining in the recording layer.
Also, among various image forming methods hitherto known, a method that requires no wet treatment and additionally has superior sensitivity, resolution and storage stability includes a method in which an image forming medium containing a photosensitive silver halide, an organic silver salt, a reducing agent and a polymerizable monomer (a polymer precursor) is subjected to imagewise exposure (such as contact exposure, projection exposure, or digital exposure), thermal amplification by heating, and then polymerization of the above polymer precursor, thus forming a polymer image (see Japanese Patent Application Laid-open Nos. 61-69062, 62-70836).
As the imagewise exposure for the above image forming medium, the digital exposure, in particular, is regarded important for the reasons that signals of image information can be processed and that the laser techniques having made great progress in recent years can be applied. It is further desired for the digital exposure to be carried out by using a semiconductor laser, from the view point of making the unit small.
Here, in order to carry out the imagewise exposure using a laser, as similarly applicable to the above optical recording medium disclosed in Japanese Patent Application Laid-open No. 57-171330, it is necessary for the above heat-developable dry silver salt to be optically sensitized to a laser wavelength region. It, however, is difficult for the dry silver salt to be sufficiently sensitized in the semiconductor laser wavelength region. There also is the disadvantage that the image forming medium having been optically sensitized has a poor storage stability.
However, as disclosed, for example, in Japanese Unexamined Patent Publications Nos. 63-18346, 63-18345, 63-19652 and 63-19653, second harmonics of laser beams (or second harmonic generation beams, hereinafter "SHG" beams") may be generated using a nonlinear optical device to carry out the imagewise exposure using the SHG beams, so that it becomes unnecessary to subject the heat-developable dry silver salt to the optical sensitization, which is difficult to effect, to the extent that the storage stability may be lowered. In that instance, however, the nonlinear optical device must be used in the recording unit, bringing about the problem that the unit is necessarily made large-sized and complicated.